Necked float zone processing of silicon rod



May 3, 1966 R. CROSBY ETAL 3,249,406

NECKED FLOAT ZONE PROCESSING OF SILICON ROD Filed Jan. 8, 1963 2 Sheets-Shee+. 1

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Q1 77 eg NECKED FLOAT ZONE PROCESSING OF SILICON ROD Filed Jan. 8, 1963 May 3, 1966 L. R. CROSBY ETAL 2 Sheets-Sheet 2 3,249,406 NECKED FLOAT ZONE PROCESSING OF SILICON ROD Lloyd R. Crosby, Beaverton, and Herbert M. Stewart,

Midland, Mich., assignors to Dow Corning Corporation,

a corporation of Michigan Filed Jan. 8, 1963, Ser. No. 250,148 4 Claims. (Cl. 23-301) nited States Patent and freedom from crystal defects heretofore unavailable,

and to the rods so produced.

The use of semiconductor materials, such as SlllCOIl and germanium, in electronic devices such as amplifiers,

rectifiers, and photoelectric cells is well known. For such uses the semiconductor material must meet unusually high standards of chemical purity as well as perfection of crystal structure. In order to refine the semiconductor to the degree of purity required it has become a standard procedure to use a crucible-free float zone process, as exemplified in US. Patent 3,030,189, issued April 17, 1962. In a typical process of this kind an elongated rod of a semiconductor material in a vertical position is heated to establish a molten zone of limited extent by means involving no contact with the rod itself, such as by induction heating. This molten zone is made to move from one end of the rod to the other one or more times, causing impurities in the rod to be driven ahead of the advancing molten zone and thus swept to the extremities of the rod. At the same time the float zone method can be used for converting a rod of polycrystalline material to a single crystal, by seeding the molten zone at one end of the rod with a monocrystal which causes the remainder of the rod to be converted to monoc'rystalline form as the molten zone progresses along the length of the rod.

An alternative method of forming elongated rods of monocrystalline semiconductor material (known generally as the Czochralski method) comprises introducing a monocrystalline seed into a pool of molten semiconductor material and withdrawing the seed from the surface of the pool at a slow controlled rate to cause the formation of a rod, as shown for example in U.S. Patent 2,631,356, issued March 17, 1953.

In the commercial production of devices employing semiconductor materials it is convenient to work with monocrystalline rods of high purity semiconductor material, from which thin slices can be cut as desired for use in various electronic devices. The power handling capabilities of certain devices using semiconductor materials, such as rectifiers, are directly proportional to the area of the section of semiconductor material, so that in these instances a rod of the largest practicable diameter is desirable. For example, whereas a slice of silicon rod of 1 inch diameter used in a rectifier has a power rating of about 450 to 500 amps, by increasing the diameter to 1.5 inches the current rating can be increased to 1,000 amps. Both the Czochralski method and the various versions of float zone processes heretofore known have certain inherent defects for producing desirably large rods having the necessary chemical purity and perfection of crystalline structure. Thus, prior to the instant invention there has existed no commercially feasible method whereby monocrystalline rods of a diameter above about 1% inches and of the necessary chemical purity and 3,249,406 Patented May 3, 1966 ably etched and viewed under a microscope. The defects appear in various forms, such as individual roughly triangular dislocations (pits) which at times are arranged linearly to form a defect known as lineage. It is, of course, desirable to reduce as much as possible the presence of such defects in the semiconductor crystal.

It has been found that in the float zone method of operation, the extent of defects in the crystal structure of the crystalline material appears to depend on the diameter of the rod, rising markedly as the diameter increases much above about 1 inch. For this reason, rods of semiconductor material having a diameter above about 1 /8 inches heretofore could not be produced by float-zoning to meet the specifications for the absence of dislocations. On the other hand, rods made by the Czochralski method, while having fewer dislocations than rods made by the float-zone refining process, were inferior to the latter in chemical purity. Thus, for example, while zone refined rods of silicon can be made having a desirably low oxygen concentration of less than 1x10 oxygen atoms per cc. of silicon, silicon rods made by the Czochralski method have at least twice the oxygen content, i.e., 2x10 oxygen atoms or more per cc. of silicon.

The present invention in its several aspects provides a method and improved apparatus for the production of high purity monocrystalline semiconductor rods and specifically, as a novel article of manufacture, a rod of high purity monocrystalline silicon having a diameter above about 1% inches.

Briefly, the invention contemplates the use of an improved induction heating coil in a more or less conventional z-one drawing process, which may be further modified in accordance with another aspect of the invention to provide that the diameter of the rod formed in the operation is larger than the minimum opening of the coil through which the rod passes in the zone forming operation.

The invention will be better understood from the following description thereof taken in conjunction with the accompanying drawings, in which:

7 FIGURE 1 is an elevational view, in partial section, of apparatus suitable for carrying out the process of the invention;

FIGURE 2 is a plan view of an improved induction heating coil having a central insert split ring for use in a float zone refining process such as that of FIGURE 1;

FIGURE 3 is a cross sectional view along the line 33 of FIGURE 2, representing a section through the split ring and one turn of the coil of FIGURE 1;

FIGURE 4 is an enlarged viewof a stage in one version of-the method of the invention showing the production of rod having a diameter larger than the minimum opening of the induction coil which is used; and

FIGURE 5 illustrates another version of the process of the invention in which the rod produced again has a larger diameter than the minimum opening of the coil, but in which there is no overall enlargement of the rod.

Referring to FIGURE 1, there is depicted zone forming apparatus of a generally conventional type which is suitable for carrying out the process of the invention.

The apparatus of FIGURE 1 comprises a base plate 10 and a dome 11 which is hermetically sealed to the base plate 10 by means of gasket 12 and clamps 13 and 14. Dome 11 is provided with a window 15 for observation of the interior thereof. Conduit 16 leads from base plate 10 to a vacuum pump (not shown) for producing, if desired, a high vacuum within the system. Dome 11 is provided on its outer surface with cooling coil 17 through which a cooling fluid can be passed for preventing excessive temperature rise within the dome. Within the dome, uprights 18 and 19 support threaded spindles 21 a and 22 which are capable of being rotated by means of motors 23 and 25 through gears 26 and 27 respectively. Upright 18 carries slidable support 28 to which is attached upper rod holder 29 by means of arm 31. Holder 29 engages the upper end of a rod 32 of semiconductor material to be processed, the lower end of which is secured in lower holder 33 which is rotatably carried by means of lower shaft 34 in base plate 10. Lower shaft 34, like all other elements extending through base plate 10, is hermetically sealed by suitable means to permit establishment of a high vacuum within dome 11 if desired. Support 28 carried on upright 18 is provided with an internally threaded bore which engages threaded spindle 21, rotation of which will cause support 28 to rise or descend as desired on upright 18. Upright 1? supports slider 36 which is also provided with an internally threaded bore engaging spindle 22, rotation of which by means of motor 25 will cause the slider to rise or descend along upright 19. Attached to slider 36 is coil 37, a specific embodiment of which is shown in detail in FIG- URE 2. As shown in FIGURE 1, coil 37 encircles the rod of semiconductor material 32. Feeding the ends of coil 37 are conductors 38 and 39 which lead to a source (not shown) of high frequency current.

During float zoning of a rod it is usually desirable to cause relative rotation in opposite directions of the portions of the rod above and below the molten zone. A number of advantages accrue from the use of such rotation. The rotation tends to cause mixing of the components present within the molten zone, thus tending to product a more uniform treatment. Again, it is'possible to cause relatively heavy impurities to be deposited under the influence of centrifugal force at the surface of the rod, from which they can be removed by suitable treatment. A further advantage of such rotation is that it indicates that the molten zone is entirely molten, Without a central solid core, and that therefore the treatment is proceeding as desired.

In order to permit such relative rotation of the portions of the rod 32, upper shaft 41, connected to upper rod holder 29, is provided with a spur gear 42 which can be driven by suitable means such as a motor (not shown) to cause rotation of the upper portion of the rod 32 above the molten zone in either direction. Similarly lower shaft 34 connected to lower rod holder 33 can be rotated by means of gear and motor 24 in either direction. In operation, relative rotation can be achieved by keeping one portion of the rod stationary and rotating the other portion or by rotating both portions of the rod in opposite directions. The latter is usually preferable since it causes the plane of shear to occur in the exact center of the molten zone, thus minimizing the tendency to cause dislocation of the relatively fluid molten material in the zone.

FIGURES 2 and 3 show in detail one embodiment of the improved induction heating coil of the invention. The embodiment shown comprises a spirally wound planar coil 51 of an electrically conducting material, in this case formed of a hollow tube 52 of silver. It will be seen from FIGURE 2 that in order to allow clearance for the return conductor 53 leading from the innermost turn of the coil, while still maintaining an approximately circular central opening, the turns of the coil are somewhat distorted on the left side thereof. We have found that the addition of a substantially circular ring 54 of electrically conducting material electrically connected as by hard solder or braze 56 to the innermost turn of the coil unexpectedly improves the crystal structure of semiconductor rods processed with the induction coil in a float zoning operation.

The reason for this unexpected improvement is not definitely known. We theorize that the symmetrical shape of the ring overcomes the unbalancing effect of the distortion in the left side in the coil of FIGURE 1 and thus tends to create a more uniform electromagnetic field within the central opening of the coil,'thus minimizing any disrupting etfect which the field may have on the crystal structure. Regardless, however, of whether or not this is the correct explanation for our improved re sults, we have found that the use of such a ring markedly reduces the incidence of dislocations and lineages in rods up to about 1 inch in diameter and, in larger rods, surprisingly improves the efliciency with which polycrystalline rods are converted to monocrystalline form by the float zoning technique.

The ring insert may be formed of any electrically conducting material which is suitable for the coil itself, such as silver and platinum. Certain conductors, such as gold and copper, tend to contaminate the rod, as will be apparent to those skilled in the art, and are therefore not preferred. As shown in FIGURE 3, the insert ring 54 may have a height approximately equal to the diameter of the tube 52 of which the coil is formed, and a thickness somewhat less than its height. These dimensions are not critical, however, and are limited only by practical considerations of size, mechanical strength, etc.

Coil 51 is preferably made from a hollow conductor rather than solid wire, since this permits a cooling fluid to be passed through the coil if desired to prevent undesirably high temperatures from forming and further in view of the fact that the outer diameter of the tube need not be any larger than that of a solid conductor, since at the high frequency used in induction heating the interior of the conductor carries little if any current.

Although the coil shown in FIGURE 2 is planar in form, the invention is not limited thereto. Central rings of the type shown can also be used in non-plan ar coils if desired, with the insert ring attached to the turn of the coil having the smallest diameter so that the ring itself makes the closest approach to a rod passing through the coil.

In order to prevent shorting out the entire innermost turn of the coil (of FIGURE 2, for example) the ring insert cannot be complete, but must be rather split at one point, as by the air gap shown or by the inclusion of a small insulating section. This gap or insulating section must of course be in such position that it serves to insulate the innermost turn from that adjacent to it, as shown in FIGURE 2.

The improved results obtained with the insert ring of the invention are demonstrated by the following examples.

EXAMPLE 1 Silicon rods having a diameter of A3 inches were float zoned with an apparatus using a four turn planar coil equipped with a silver insert ring having an inner diameter of one inch, such as that shown in FIGURE 2. The results of this work were evaluated by calculating for each of the rods a Q or quality number by means 0 the equation where Q=Quality or Q number L=Length or longest lineage, mm.

N =Number of lineages T=A constant based on the type of lineage: (1) single;

(2) multiple; (3) gross.

As used in the above equation, multiple lineage indicates more than one lineage in the specimen, while gross lineage indicates the presence of parallel lineage lines closer than mm. It will be seen that increasingly serious defects produce larger Q numbers, so that a low Q number is desired as indicative of a high quality rod.

Slices from the rods were prepared for inspection under a microscope in the following manner: The surface of the slice to be examined was flooded with a mixture of 3 parts by weight nitric acid, reagent grade 2 parts acetic acid, reagent grade (99.7%), and 2 parts hydrofluoric acid, electronic grade (49%). The solution was left on the surface for 2 minutes to produce a mirror finish. The solution was then washed away with deionized water and the surface of the specimen was dried with methyl alcohol. The specimen was then etched by applying thereto a mixture of two parts :by Weight of a solution of 50 g. of chomium trioxide, reagent grade, in 100 ml. of deionized water and one part hydrofluoric acid, electronic grade (49%). The specimen was etched for from to 12 minutes at room temperature and then .flooded with deionized water to remove the etching solution and finally washed with methyl alcohol.

For 26 rods which were zoned with the ring insert, the average Q-value was 2.8. By comparison, for 36 rods which were zoned using a coil having the same inner diameter (1 inch) but no insert ring, the average Q-value was 6.9. These data demonstrate that the presence of the ring definitely improved the crystalline properties by suppressing the formation of lineages.

Another obstacle which has prevented the commercial production of fioat zoned rods larger than about 1 /3 inches in diameter is the fact that in these large sizes the surface tension of the material in the molten zone, which typically has a height equal to the diameter of the rod, is not suflicient to contain within the zone the relatively fluid molten material, which thus has a tendency to sag and drip. Still another obstacle stems from the fact that it becomes increasingly difficult to supply sufficient heat to obtain liquefaction of the material at the center of relatively large (over 1%; inch diameter) rods. For these reasons, and entirely independent of the quality of the product in terms of the presence of dislocations and lineages, rods of 1% inch diameter could be obtained with difficulty only in the laboratory and not in commercial production, while float-zoned rods of 1 /2 inch diameter or more could not be made at all with the methods heretofore used.

In another aspect of the invention, crystalline rods of semiconductor material, e.g., silicon, in sizes greater than about 1 /8 inches can be made in a commercially feasible manner by means of a technique which involves passing the semiconductor rod through an induction heating coil, such as that of FIGURE 2, which has a central opening smaller than that of the finished rod. The method can be carried out in one of two ways as depicted in FIG- URES 4 and 5. In one case (FIGURE 4) the rod is caused to grow in diameter by pile zoning or squeeze zoning, i.e., feeding a relatively small rod through the central opening of the coil while bringing the ends of the rod toward each other, causing the thickness of the rod to increase. In the vicinity of the coil the silicon is molten and as the rod is fed into the zone at a carefully adjusted rate, the molten material spreads below the coil to form a rod of a diameter larger than that of the original rod. The rate at which the ends of the rod are fed toward each other relative to the rate of advancement in FIGURE 5, the diameter of the rod before and after processing is not changed by the fioat zoning treatment. During treatment, however, the rod is causedto pass through a coil having a diameter smaller than the initial (and final) diameter of the rod. The rod is capable of going through a ring or coil having a smaller diameter than itself by reason of the fact that it is molten in the vicinity of the coil and is pinched to a narrow neck by the field created by the coil, as shown in FIGURE 5, as the ring travels in one direction or the other to sweep the entire length of the rod.

We have found that the use of an induction coil having an internal diameter smaller than the finished rod permits the production of rods of relatively large diameter (that is, 1 /2 inches or more). The relatively short distance from the inner turn of the coil to the center of the rod facilitates transfer of energy to this point, thus insuring complete melting of the rod across the entire section. In addition, the field of the coil which creates the pinched neck appears to assist in maintaining the molten material in position within the zone, thus obviating difficulties heretofore encountered resulting from the tendency of this material to fall away under its own Weight.

A further advantage of the use of a coil of such rela -tively small diameter under certain conditions is that rods produced in this manner show an unexpected improvement in radial resistivity of slices taken across the rod. Again, the explanation for the improvementobserved by using this technique is not definitely known. It can be theorized, however, that the closer coupling of the ring with the central portion of the formed rod creates a more uniform condition within the molten zone, which manifests itself in improved radial resistivity, such that the resistivity across a diameter of a slice taken from such a rod can be held to a variation from the maximum value of not more than 20%.

The advantages of operating in accordance with this aspect of the method of the invention are demonstrated by the following examples.

EXAMPLE 2 A silicon rod having an initial diameter of /8 inch was pileor squeeze-zoned in accordance with the method of FIGURE 4, using a coil with a ring insert having an inner diameter of 25 mm. to obtain a monocrystalline rod with a diameter of 1.67-1.69 inches. measurements were taken at 3 mm. intervals across the lower (seed) end and the top end of the crystal by means of a 4-probe method described by Valdes (Proceedings of the molten zone is obviously dependent on the relative I cross sectional areas of the rod before and after expansion and can be easily calculated on this basis. For example, if the ends of a-rod are kept fixed relative to each other during float zoning (i.e., no squeezing or piling is used), the diameter of the rod at a given point remains constant before and after the molten zone has swept through that point. On the other hand, assume that it is desired to double the diameter of a rod by squeeze zoning, and further assume that the molten zone is caused to move upwardly at a convenient speed, say, 2 mm. per minute. It will be seen that if the portion of the rod above the molten zone is moved downwardly at the rate of 8 mm. per minute the diameter of the rod in the solidifying molten zone will increase to double that of the initial rod. This follows from the fact that the amount of material leaving the molten zone must be equal to that entering, since no accumulation occurs within the zone.

In another version of the method of the invention shown of the I.R.E., February 1954, pp. 420427). At the top end, the measurements ranged from 10 1 to ohm centimeters, representing a variation of 12% of the maxi- I mum value, while at the seed end the figures ranged from 107 to ohm centimeters, representing a variation of 17%. The oxygen content of this rod was less than 1 10 atoms per cc. of silicon.

The product of Example 2, i.e., a monocrystalline silicon rod having a diameter over 1%; inches (1.65 inches in this case), having at the same time a variation of less than 20% in radial resistivity and an oxygen content of less than 1 1O oxygen atoms per cc. ofsilicon represents a novel product heretofore impossible to obtain. Although monocrystalline rods of this diameter can be made by the Czochralski method, such rods invariably have oxygen contents at least twice as high (i.e., 2x10 atoms per cc. of silicon) and radial resistivities which vary by 40% or more across a diameter.

EXAMPLE 3 Twelve silicon rods were squeeze zoned from an original diameter of 22 mm. to a final diameter of 40 mm. (1.6 inches) using planar 4-turn coi-ls having an inside diameter of 25 mm. In six of the runs the coil contained an insert ring, while in the remaining six runs the insert ring was omitted.

Resistivity With the insert ring present, three of the rods were converted to monocrystalline form throughout 100% of their lengths, while another rod was converted throughout 95% of its length. Overall, 66% of the combined lengths of the six rods was converted to commercially useable monocrystalline material after two passes. By contrast, in the absence of the insert ring, but under otherwise identical conditions, none of the rods was entirely converted, the maximum being 60%, and overall only 25% of the combined lengths of the rod was converted to monocrystalline form.

Although the method of the invention involving enlarging the diameter of the rod is preferably carried out with a coil equipped with a central insert ring, it is not limited thereto. Even without such a ring insert, the method permits the commercial production of large diameter rods which meet commercial standards of quality, as shown in the following example.

EXAMPLE 4 A polycrystalline silicon rod having a diameter of 1 /2 inches (38 mm.) was successfully converted to a monocrystalline rod of the same diameter by float zoning, using a planar coil having an internal diameter of 29 mm.

- The start of a run in which the diameter of the feed rod non-conductive silicon rod. When the rod becomes heated sufiiciently to become electrically conductive under the influence of the field generated by the coil, the coil is temporarily removed from its position adjacent the end of the rod and the foil is removed, as by jarring it loose. The coil is then repositioned and inductive heating of the end of the rod is continued until a drop of molten material is seen to be produced. At this point a radial resistivity values and dislocation densities within the commercially acceptable range and in addition had only a small amount of lineage, ranging from none at the seed end to slight at the top end.

The foregoing detailed description has been given for clearness and understanding only, and no unnecessary limitations should be understood therefrom, as modifications will be obvious to those skilled in the art.

What is claimed is:

1. In the method of float-zone processing a cylindrical rod of crystalline semiconductor material comprising heating to the melting point a cross-sectional zone of said rod by means having no contact with said molten zone and causing said molten zone to be displaced longitudinally along said rod, said molten zone being created by means of an electric current of high frequency, the improvement comprising moving longitudinally with respect to said rod from one end of said rod to the other an encircling electrically conductive coil having an internal diameter smaller than the diameter of all cylindrical portions of said rod outside of said molten zone, while passing through said coil a high frequency current, said moving of said coil with respect to said rod being efi'ectuated by the electromagnetic field of said coil pinching said molten zone to a neck of a size capable of passing through said coil.

2. The method of claim 1 wherein said semiconductor material is silicon and said rod has a diameter not less than 1% inches. 1

3. The method of claim 1 in which said coil has a plurality of turns and is provided with a substantially circular split ring electrically connected to a turn of said coil having the smallest diameter.

4. The method of claim 3 wherein said semiconductor material is silicon and said rod has a diameter not less than 1% inches.

References Cited by the Examiner UNITED STATES PATENTS 2,481,008 9/1949 Gagliardi et al. 219-10.79 X 2,481,071 9/1949 Bowlus 21910.79 X 2,485,843 10/ 1949 Pinkney 2l9-10.79 X 2,825,120 3/1958 Smith 16 1-177 2,972,525 2/1961 Emels 23-273 2,990,259 6/ 1961 Moody et al. 3,002,821 10/1961 Haron. 3,023,091 2/1962 Smith 2330-1 3,038,239 6/1962 Moulds 161-177 3,046,100 7/1962 Siemons et al. 3,065,062 11/1962 Enk et al. 3,100,250 8/1963 Herczog et al. 23-273 X FOREIGN PATENTS 908,370 10/1962 Great Britain.

NORMAN YUDKOFF, Primary Examiner.

RICHARD M. WOOD, Examiner.

G. HINES, L. H. BENDER, Assistant Examiners. 

1. IN THE METHOD OF FLOAT-ZONE PROCESSING A CYLINDRICAL ROD OF CRYSTALLINE SEMICONDUCTOR MATERIAL COMPRISING HEATING TO THE MELTING POINT A CROSS-SECTIONAL ZONE OF SAID ROD BY MEANS HAVING NO CONTACT WITH SAID MOLTEN ZONE AND CAUSING SAID MOLTEN ZONE TO BE DISPLACED LONGITUDINALLY ALONG SAID ROD, SAID MOLTEN ZONE BEING CREATED BY MEANS OF AN ELECTRIC CURRENT OF HIGH FREQUENCY, THE IMPROVEMENT COMPRISING MOVING LONGITUDINALLY WITH RESPECT TO SAID ROD FROM ONE END OF SAID ROD TO THE OTHER AN ENCIRCLING ELECTRICALLY CONDUCTIVE COIL HAVING AN INTERNAL DIAMETER SMALLER THAN THE DIAMETER OF ALL CYLINDRICAL PORTIONS OF SAID ROD OUTSIDE OF SAID MOLTEN ZONE, WHILE PASING THROUGH SAID COIL A HIGH FREQUENCY CURRENT, SAID MOVING OF SAID COIL WITH RESPECT TO SAID ROD BEING EFFECTUATED BY THE ELECTORMAGNETIC FIELD OF SAID COIL PINCHING SAID MOLTEN ZONE TO A NECK OF A SIZE CAPABLE OF PASSING THROUGH SAID COIL. 